1. Field of the Invention
The invention relates to a probe for connection to measuring devices, especially to oscilloscopes, with high-precision, DC-voltage measurement.
2. Discussion of the Background
In order to keep pace with the ever-increasing bandwidths of modern bus systems, modern active probes also have to provide ever-increasing bandwidths. The amplifiers required for this purpose must be continuously optimized in the direction towards larger bandwidths (for example, 5 GHz and above). Statistical properties, such as DC-voltage amplification and offset, must be sacrificed to the demand for larger bandwidth. However, alongside the measurement of fast signals, a very precise measurement of DC-voltages is often also desirable in practice. To measure DC-voltages as accurately as possible, the current practice is initially to match the probe to an earth potential and to eliminate the DC-voltage offset via the position adjustment on the basic device. After this, the DC-voltage to be measured can be determined in an offset-free manner. This method is costly, does not eliminate the amplification error and cannot be used in the case of an AC voltage coupling. A parallel, high-precision DC-voltage measurement, which can be displayed independently of the test signal, would be desirable.
With currently available broadband probes for oscilloscopes, DC-voltages can only be measured very inaccurately, typically with an amplification error of 2% and an offset of several millivolts. Added to this is the error from the input resistance of the oscilloscope (typically 1%), the amplification error of the input amplifier and of the analog/digital converter in the oscilloscope (typically 1%) and the associated offsets. These errors can in fact be calibrated; however, the DC-voltage accuracy is unsatisfactory, because the calibration depends, for example, upon channel, temperature and selected amplification and position. Moreover, non-linearities cannot be calibrated at all.
Accordingly, U.S. Pat. No. 6,856,126 B2 discloses a conventional probe for broadband measurement of high-frequency test signals. The entire signal is picked up with a probe tip. The entire signal is amplified by the same amplifier and transmitted to the measuring device by the same line. The problems described of inaccurate DC-voltage measurement are very clearly evident. A compensation of non-linear interference of the DC-voltage components is not possible.